opencode: support directory-based configuration for commands, agents, and themes

The `commands`, `agents`, and `themes` options now accept either an
attribute set (existing behavior) or a path to a directory containing
multiple files. When a directory path is provided, it is symlinked to
the appropriate `$XDG_CONFIG_HOME/opencode/` subdirectory.

This change aligns with the existing `skills` option implementation and
provides a more convenient way to manage multiple configuration files
without needing to define each one individually in Nix.

tests/modules/programs/opencode/themes-bulk/light-theme.json

@@ -0,0 +1,8 @@
+{
+  "$schema": "https://opencode.ai/theme.json",
+  "name": "light-theme",
+  "colors": {
+    "primary": "#ffffff",
+    "secondary": "#f3f3f3"
+  }
+}